An electric generator comprises a rotor arranged inside a stator. The rotor turns at a frequency
                                                        f              mech                        ⁡                          [              rpm              ]                                =                                    2              ·                                                f                  el                                ⁡                                  [                  Hz                  ]                                            ·              60                        P                          ,                            (        1        )            
Where P is the number of poles of the machine and fel is the electrical frequency of the network. Electric generators are typically synchronous machines, so the electrical frequency would be fel=50 Hz or 60 Hz. Consequently, the rotor of a two-pole electric generator (P=2) turns at 3000 rpm in 50 Hz networks and at 3600 rpm in 60 Hz networks. The rotor of a four-pole electric generator (P=4) will turn at 1500 rpm in 50 Hz networks and at 1800 rpm in 60 Hz networks.
The stator comprises a stator core with a plurality of slots. The slots are arranged in between teeth. Both the teeth and the slots are essentially parallel to the axis of the rotor. The rotor is arranged inside a cylindrical bore in the center of the stator core. That bore is often referred to as the stator bore. The stator slots are open towards the stator bore.
When the electric generator is a three-phase synchronous machine, the number of slots is often a multiple of six. Typically, a stator comprises 36, 42, 48, 54, or 60 slots and an equal number of teeth.
The winding of a stator is typically made up of stator bars arranged inside the slots. Most windings of large electric generators are double-layer windings. A double-layer winding provides a top position and a bottom position inside each slot. In stators with double-layer windings, a coil comprises two stator bars and typically occupies the top position of a first slot and the bottom position of a second slot.
Each stator bar comprises conductors which carry the current. On one side of the stator, the two stator bars of the same coil leave their stator slots and are electrically connected. This pairwise electrical connection is commonly referred to as the end-winding and electrically closes the circuit. On the other side of the stator, a lead is brought out from each stator bar. The leads are then used to complete the stator winding by connecting the stator bars among each other. They leads may also be used to connect the three-phase output of the generator.
Different schemes exist to connect the coils of a stator winding. When the winding is arranged as a lap winding, several of the coils of each phase are electrically connected in series.
A modest increase in the mechanical frequency of the rotor often yields a reduction in size and improved efficiency of an electric generator. An increased frequency fmech of the rotor would, however, also result in an increase of the electrical frequency fel of the three-phase output of the generator. To vary the electrical frequency fel, a combination of an electric generator and of frequency conversion through AC/AC conversion provides a technical solution. The application WO 2006/103159 discloses such a combination of an electric generator and of AC/AC conversion. FIG. 3b of WO 2006/103159 discloses a polygonal winding, with a plurality of series-connected (lap-wound) coils. The embodiment shown on FIG. 3b of WO 2006/103159 shows a winding with 54 slots and 18 bidirectional switches. In other words, the polygonal winding of FIG. 3b consists of groups of three series-connected coils. Each of these groups provides one phase of the polyphase (18-phase) output of the winding. To generator three-phase output, the groups of series-connected coils are connected to three busbars through a total of 18 bidirectional switches.
The arrangement of 18 bidirectional switches feeding three busbars is also referred to as an 18×3 matrix converter. The 18×3 matrix converter provides AC/AC conversion by connecting and disconnecting the individual phases of the polyphase output to and from the three-phase output of the electric generator.
The matrix converter of WO 2006/103159 converts the electrical frequency of the three-phase output of the electric generator downwards. The embodiment disclosed by WO 2006/103159 can thus be used to convert 60 Hz three-phase output to 50 Hz three-phase output. To generate 60 Hz output, an electrical frequency fel of 100 Hz needs to be converted to 60 Hz.
According to equation (1) the rotor of a two-pole electric generator (P=2) would have to rotate at fmech=6000 rpm in order to provide fel=100 Hz. In gas-powered plants, the electric generator is driven by a gas-turbine or steam turbine. A mechanical frequency of fmech=6000 rpm would then require not only modifications of the electric generator but also a gas-turbine engine designed for fmech=6000 rpm. There is thus a need to obtain three-phase output at fel=60 Hz from an electric generator whose rotor turns at fmech=3000 rpm.
Equation (1) implies an electrical frequency fel of 100 Hz can readily be obtained by increasing the number of poles from P=2 to P=4. The electrical frequency of 100 Hz could then be converted to fel=60 Hz through a matrix converter. It is, however, not straightforward to modify the stator winding accordingly. A change from a two-pole rotor to a four-pole rotor also impacts on the symmetry of the stator winding. Opposite sides of the stator winding will now experience the same value and direction of magnetic flux. Corresponding coils on opposite sides of the stator should thus either be connected in parallel or in series. If opposite coils were not connected together to form one phase, then twice as many bidirectional switches would be necessary to maintain the same number of switches per pole. In applied to the winding shown on FIG. 3b of WO2006/103159, the result would be a winding with 36 bidirectional switches. There would also be a fractional number of (1.5) series-connected coils per bidirectional switch.
If corresponding coils on opposite sides of the stator were connected in parallel, the number of series-connected coils per bidirectional switch would be cut in half. Consequently, the three-phase output voltage would be lower by a factor two and the current would be higher by the same factor. Since the forces between conductors follow the square of the current, a winding scheme which delivers twice the current would need additional mechanical reinforcement and an especially got cooling system to compensate for the losses causes by the high current.
If the bidirectional switches were made of thyristors, any misalignment of the switching of those thyristors would create additional difficulty. In particular, any delay between the thyristors arranged on opposite sides of the winding would result in asymmetrical (eddy) currents the rotor surface.
To avoid the above issues with thyristors, a plurality of long conductors could be used to bridge opposite parts of the winding. The plurality of long conductors could actually be used to connect opposite parts of the winding in series or in parallel. Each of those long conductors would span half the circumference of the stator. The scheme shown on FIG. 3b of WO2006/103159 would require nine such conductors and nine bidirectional switches. Each of those conductors would need mechanical support and add another part of the stator winding that may eventually fail. That solution would be complex and impair the reliability of the electric generator.
The present disclosure is oriented towards providing the aforementioned needs and towards overcoming the aforementioned difficulties.